1991 Yamaha 4.3 YEMS gremlins

[thumbdoctor]

Yes, I know, I have bought a good anchor or insurance submarine. Anyways, I like a good challenge so I bought this barge. The Yamaha 4.3 YEMS stern drive is in good shape (Even shifts well !) and I have rebuilt the sour Rochester 4MV. The problem I am experiencing is the RPM is limited to about 2400~ 2600 in neutral or drive out of the water with good cold water supply. The issue sounds like an ignition strategy (rev limiter) programmed into the engine control module preventing over rev in neutral or when one of the tell tale alarms are engaged. Can anyone confirm this?. I have a Clymer service manual on order so I can verify the wiring schematics and familiarize myself with the YEMS operation but I'm afraid the Clymer may not go in-depth enough to cover this issue. The engine skip is definitely ignition (Proved by tracing secondary voltage with a dual trace Oscilloscope) so I surmise that there is a Fail Safe programmed into the system. Any help would be much appreciated.

 

[Don S]

Re: 1991 Yamaha 4.3 YEMS gremlins

List of possible causes of low WOT (Wide Open Throttle) rpm.

In no particular order

Engine Won't Reach Operating RPM. Check

1. Fuel condition. Type and Octane possibly old fuel
2. Propeller pitch or diameter, damaged blades
3. Restricted fuel pickup tube or anti siphon valve Fuel System Test
4. Crankcase oil volume, high oil level can cause aerated oil and lifter collapse
5. Marine growth on hull and outdrive
6. Wrong gear ratio in outdrive
7. Restricted carburetor air intake (clogged flame arrestor)
8. Restricted exhaust system (broken exhaust shutters/flappers) in engine transom shield or drive
9. Poor cylinder compression Compression Test
10. Carburetor defective, or wrong type.
11. Fuel pump pressure and vacuum
12. Boat overloaded, improperly loaded, or improperly trimmed.
13. Engine Overheating
14. Engine timing and ignition system operation
15. Remote control cables and linkage for proper travel to open throttle plates fully.

 

[thumbdoctor]

Re: 1991 Yamaha 4.3 YEMS gremlins

List of possible causes of low WOT (Wide Open Throttle) rpm.

In no particular order
Thanks for the "Heads Up" Don !
Engine Won't Reach Operating RPM. Check

1. Fuel condition. Type and Octane possibly old fuel Fresh 87
2. Propeller pitch or diameter, damaged blades Not in the pond yet, cold water supplied and does the same in drive or reverse
3. Restricted fuel pickup tube or anti siphon valve Fuel System TestRunning from a 5 gallon Jerry can
4. Crankcase oil volume, high oil level can cause aerated oil and lifter collapse Fresh oil & filter, correct dipstick and level. Oil pressure good.
5. Marine growth on hull and outdrive Out of water on trailer
6. Wrong gear ratio in outdrive Stock OEM Yamaha ratio
7. Restricted carburetor air intake (clogged flame arrestor) Arrester clean and tried without it
8. Restricted exhaust system (broken exhaust shutters/flappers) in engine transom shield or drive Manifolds, risers and flappers working. Plenty of exhaust exiting leg and prop.
9. Poor cylinder compression Compression TestCompression 180 PSI on all cyls using Snap On tester.
10. Carburetor defective, or wrong type. Just rebuilt by myself (40 years experience with Rochesters, Webers, Mikunis, Holleys and Carters).
11. Fuel pump pressure and vacuum Fuel pump has 15 inches of vacuum and delivers 2 pints in 15 seconds @ 4 PSI.
12. Boat overloaded, improperly loaded, or improperly trimmed. Refer to #2
13. Engine Overheating Engine running indicated 180 F checked with melt stick on thermostat housing and infra red thermometer.
14. Engine timing and ignition system operation Ignition base timing @ 8 degrees BTDC (Jumper attached) 700 RPM electronic retard & advance working as designed. Strong blue spark at all new spark plugs.
15. Remote control cables and linkage for proper travel to open throttle plates fully.

Cables adjusted correctly with no binding. Tried operating carb directly with cable disconnected.

Other opinions?

 

[Bondo]

Re: 1991 Yamaha 4.3 YEMS gremlins

Ayuh,... Contact Capt. Ken, at Nelson Marine...